// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart.core;

/**
 * 保留字[:null:]表示该类唯一的实例化对象。
 *
 * 当试图继承或实现Null类的时候，会产生一个编译时错误。
 */
class Null {
  factory Null._uninstantiable() {
    throw new UnsupportedError('class Null cannot be instantiated');
  }

  /** 返回字符串`"null"`。 */
  String toString() => "null";
}
